Relative Velocity

When two bodies (say A and B) are moving, the velocity of A as seen by B depends on their actual velocities. This velocity is termed as 'velocity of A relative to B' with notation VAB.

Where VA and VB are velocity of A and B respectively. The subtraction is vectorical. For example, if Car A is moving 3km/h north while car B is moving 4km/h east, then the magnitude of VAB or VBA is 5km/h but NOT 1km/h.

Another example, when two cars are moving in the same direction with the same speed, the drivers in both cars would find that they have no relative motion at all.